An integral composite membrane is provided which is useful in electrolytic processes and other chemical separations.
Ion exchange membranes (IEM) are used in polymer electrolyte fuel cells as solid electrolytes. A membrane, located between a cathode and an anode of such a fuel cell, transports protons formed near the catalyst at the hydrogen electrode to the oxygen electrode, thereby allowing a current to be drawn from the fuel cell. These polymer electrolyte fuel cells are particularly advantageous because they operate at lower temperatures than other fuel cells. Also, these polymer electrolyte fuel cells do not contain any corrosive acids which are found in phosphoric acid fuel cells. In these type fuel cells, there is a need to eliminate the bulk transfer of reactants from one electrode to the other, i.e. fluid percolation.
Ion exchange membranes are also used in chloralkali applications to separate brine mixtures to form chlorine gas and sodium hydroxide. For best performance, it is preferred that the membrane selectively transport the sodium ions across the membrane while rejecting the chloride ions. Also, the ion exchange membrane must eliminate bulk transfer of electrolytic solution across the membrane, i.e. fluid percolation.
Additionally, IEMs are useful in the areas of diffusion dialysis, electrodialysis and in pervaporation and vapor permeation separations. IEMs may also be used for selective transport of polar compounds from mixtures containing both polar and non-polar compounds.
IEMs must have sufficient strength to be useful in their various applications. Often, this need for increased strength requires that an IEM be made relatively thick in cross section, or that the IEM be reinforced with woven fabrics (macro-reinforcements), both of which decreases the ionic conductance of the IEM. Additionally, conventional IEMs exhibit inherent dimensional instability due to the absorbance of solvents, such as water, for example. Such dimensional instability renders conventional IEMs substantially ineffective for many commercial applications.
U.S. Pat. No. 3,692,569 relates to the use of a coating of a copolymer of fluorinated ethylene and a sulfonyl-containing fluorinated vinyl monomer on a fluorocarbon polymer that was previously non-wettable. The fluorocarbon polymer may include tetrafluoroethylene polymers. This coating provides a topical treatment to the surface so as to decrease the surface tension of the fluorocarbon polymer. U.S. Pat. No. 3,692,569 provides for a fluid percolating structure.
U.S. Pat. No. 4,453,991 relates to a process for making articles coated with a liquid composition of a perfluorinated polymer, having sulfonic acid or sulfonate groups in a liquid medium, by contacting the polymer with a mixture of 25 to 100% by weight of water and 0 to 75% by weight of a second liquid component, such as a low molecular weight alcohol, in a closed system. Such a process provides for a multi-layered structure.
U.S. Pat. No. 4,902,308 relates to a film of porous expanded polytetrafluoroethylene (PTFE) having its surfaces, both exterior and internal, coated with a metal salt of perfluoro-cation exchange polymer. Such a composite product is permeable to air. The air flow of such a structure, as measured by the Gurley densometer ASTM D726-58, is about 12 to 22 seconds. Therefore, this structure provides for fluid percolation.
U.S. Pat. No. 5,082,472 relates to a composite material of a microporous membrane, such as porous expanded PTFE, in laminar contact with a continuous ion exchange resin layer, wherein both layers have similar area dimensions. Surfaces of internal nodes and fibrils of the expanded PTFE may be coated, at least in part, with an ion exchange resin coating. The expanded PTFE layer of this composite membrane imparts mechanical strength to the composite structure. However, the interior of the expanded PTFE membrane is unfilled so as to not block the flow of fluids. Therefore, U.S. Pat. No. 5,082,472 provides for fluid percolation.
U.S. Pat. No. 5,094,895 and 5,183,545 relate to a composite porous liquid-permeable article having multiple layers of porous expanded PTFE, which are bonded together, and which have interior and exterior surfaces coated with an ion exchange polymer. Such a composite article is particularly useful as a diaphragm in electrolytic cells. However, diaphragms are inherently percolating structures.
Japanese Patent Application No. 62-240627 relates to a coated or an impregnated membrane formed with a perfluoro type ion exchange resin and a porous PTFE film to form an integral structure. The resulting composite is not fully occlusive. Furthermore, the teachings of this application do not provide for permanent adhesion of the ion exchange resin to the inside surface of the PTFE film.
There remains a need for a strong, integral composite ion exchange membrane, having long term chemical and mechanical stability.
The present invention is an advancement over presently known ion exchange membranes. In one embodiment of the present invention, this is accomplished by providing a composite membrane comprising an expanded polytetrafluoroethylene (PTFE) membrane having a porous microstructure of polymeric fibrils. The composite membrane is impregnated with an ion exchange material throughout the membrane. The impregnated expanded polytetrafluoroethylene membrane has a Gurley number of greater than 10,000 seconds. The ion exchange material substantially impregnates the membrane so as to render an interior volume of the membrane substantially occlusive.
The expanded PTFE membrane may comprise a microstructure of nodes interconnected by fibrils.
The ion exchange material may be selected from a group consisting of perfluorinated sulfonic acid resin, perfluorinated carboxylic acid resin, polyvinyl alcohol, divinyl benzene, styrene-based polymers, and metal salts with or without a polymer. The ion exchange material may also be comprised of at least in part a powder, such as but not limited to, carbon black, graphite, nickel silica, titanium dioxide, and platinum black.
A purpose of the present invention is to provide an improved alternative to the macro-reinforcement of ionomer materials.
Another purpose of the present invention is to provide an ion exchange membrane having a single integral structure that does not allow for fluid percolation.
The foregoing and other aspects will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing figures.